A viewing device of this kind is already known by the French patent FR-2.484.658 and enables binocular observation of a single real or overhead image with short focal length. The reduced space requirements and weight of such a viewing device in relation to those known previously enable low cost production of a light and portable instrument. This viewing device is therefore intended for being hand-held or attached to the user's head via a support, for instance a helmet, for civilian or military usage.
As a rule, night-time viewing devices comprise of the following elements:
an objective lens which forms an image of the scene observed on the photocathode of a luminance intensifier tube, PA1 the luminance intensifier tube, PA1 an ocular (eye-piece) arranged in front of the output screen of the luminance intensifier tube, PA1 in the case of binocular viewing devices, an image splitting and transmitting device capable of creating images which can be observed by both of the user's eyes. PA1 an excessive price when they possess two light intensifier tubes or when, for compactness reasons, the optical path is too complex, PA1 an excessive weight, PA1 the centre of gravity located quite ahead of the eyes' position, causing excessive tiredness of the user, PA1 the fields of observation of both eyes are asymmetrical or limited, PA1 the adjustments are too complex, PA1 the system does not make enough space for the observer's nose. PA1 the optical image splitter is a semi-reflecting cube or blade, PA1 the viewing device comprises an electrical energy source capable of supplying a light-emitting diode intended for night-time lighting, as well as the luminance intensifier tube, using a single electronic board ensuring conversion of the supply voltage, antiglare safety of the said tube and disconnection of the assembly via a related external switch, PA1 the voltage of the electrical energy source can be 1.5 V or 3 V, PA1 the switch can be operated manually or automatically whereby contact is made or suppressed with the support of the viewing device, according to whether the said device in connected or disconnected, by toggling to an articulation used as an interface in relation to the support, PA1 it comprises a light-emitting diode forming a make-up lighting system actuated by a switch comprising a neutral position, a position in which there is a permanent contact in the absence of a voluntary return, a position in which there is an intermittent contact with automatic return, PA1 the viewing device comprises, a rear casing containing all of the following optical elements: the optical image splitter, the three mirrors and the lens system making up the optical image transmitting system, as well as the first and second oculars and, a front casing supporting the luminance intensifier tube, the objective and all the following electric or electronic components: the switch, the electrical energy source, the diode, the tube safety device, the board and the safety light-indicators, as well as the articulation means making up an interface in relation to the support, PA1 the rear and front casings are made of a high pressure moulded plastic material, to form a single piece, individually from one another and assembled at a later stage, after placement of their respective constitutive elements, in relation to a tight common joint plane.
Various types of night-time viewing devices are today in operation and can be differentiated from one another by the number, the type and the arrangement of the light intensifier tube(s) or by the image splitting and transmitting device.
All these devices exhibit one or several shortcomings, more or less important, such as:
The present invention, in order to remain economical, therefore only uses one luminance intensifier tube and brings a new solution to the problem of image splitting and transmission. This solution, while remaining very economical, obviates all operation shortcomings of the equipment existing today. It also exhibits the advantage of providing the observer with a very widened field of vision, close to the normal field of vision of the eye. This last point is extremely user-friendly and enables the observer to avoid obstacles when moving.
It should be noted here that the main difficulty to overcome for image splitting and transmission is the necessity to observe a small diameter screen, as they are now available, with a wide angle. This requires the usage of oculars whose focal length is slightly greater than the screen diameter.
When the arrangement adopted for the whole device is a symmetrical one, i.e. when the light intensifier tube is located in the centre position, halfway between the oculars' axes, it is not possible, because of the distance between both eyes, to split and to transmit the image using simple mirrors while arranging the tube as close to the eyes as possible. On the contrary, it is necessary to insert image optical transmitting, and possibly, rectifying devices which can be heavy and cumbersome.
If the arrangement adopted is an asymmetrical one, the tube is than located on one of the eyes' axis and can be observed directly by this eye through an ocular. It is relatively easy to obtain, for this eye, a 40.degree. to 50.degree. field of vision. In such a case, image splitting is performed by a prism or a semi-transparent mirror in order to send the image to the other eye.
For the other eye to be able to observe this image with the same apparent angle as the first, an ocular with the same focal length must be used, arranged in such a way that the image to be observed lies in its focal plane. Taking into account the existing distance between the observer's eyes, it is not possible to place this ocular directly in front of the second eye.
A solution, subject of a French patent FR-2.484.658, consists in placing this second ocular close to the image splitting prism and therefore at quite a distance from the second eye. This efficient solution possesses nevertheless the inconvenience of reducing the total field seen by the second eye, in spite of the use of glass prisms spread over the whole path in order to artificially reduce the optical distance.
The solution also had the defect of introducing an important weight in the system and not to leave much space for the user's nose.